In recent years, the pharmaceutical, biotechnology, chemical and related industries have increasingly adopted micro-chamber and channel structures for performing various reactions and analyses. The benefits of these structures include miniaturization, reduction in space and cost of reagents and enable one to perform a large number of reactions without human intervention, either in parallel or in series (i.e., one after the other).
Microfluidic devices are by far the most promising candidates to realize a micro-TAS (Micro Total Analysis System). In general, all attempts in this direction can be characterized in two ways: according to the forces responsible for the fluid transport and according to the mechanism used to direct the flow of fluids. The former are referred to as motors. The latter are referred to as valves, and constitute logic or analogue actuators, essential for a number of basic operations such as volumetric quantitation of fluids, mixing of fluids, connecting a set of fluid inputs to a set of fluid outputs, sealing containers (to gas or to liquids passage according to the application) in a sufficiently tight manner to allow fluid storage, regulating the fluid flow speed.
As motors, the prior art disclose a variety of solutions, including electro-kinetic and electro-osmotic transport, mechanical micro-pumps, external pressure, acoustic energy, and the centripetal force. The present invention is concerned primarily, but not exclusively, with the category of centripetal devices. Accordingly, a summary of some of the prior art related to centripetal devices includes: Yamaji et al. (EP00392475A2) and Takase et al. (EP00417305A1) disclose a liquid sample analyser based on a rotating disk; Kellogg et al. (U.S. Pat. No. 6,063,589/WO0187485A2) and Mian et al. (U.S. Pat. No. 6,319,469, US21055812A1) disclose devices and methods for using centripetal acceleration to drive fluid movement in a microfluidic system; Kopf-Sill et al. (U.S. Pat. No. 5,160,702) teaches an analyser with improved rotor structure; and Gordon (U.S. Pat. No. 5,892,577, U.S. Pat. No. 6,256,088, U.S. Pat. No. 6,339,473) teaches an apparatus and method for carrying out analysis of samples.
Devices with the ability to regulate fluid flow through the use of valves are known in the prior art, and differ in their ability to provide real-time control and analogue adjustment of the fluid flow. As an example, some valves have the ability to regulate fluid flow in an analogue manner, like hot water taps, some valves switch between on-off states and vice versa, like irrigation actuators, some valves have a single on-off transition, like electrical safety switches, or off-on-transition, like safety valves in pressurized circuits.
Prior art microfluidic valve devices suffer from the drawback of high cost per valve, as well as the integration scale and complexity that can be achieved. Unfortunately, the reliability of most prior art devices within the meso-scale range is suspect. Further, alteration of sample material by the components of the valve and function of the valve has contributed to their unreliable nature and failure to produce a micro-analytical device with reproducible results. The design of prior art valving devices makes their manufacturing cost and complexity unsuitable for their cost effective use within micro-analytical devices that are “throw-away” and mass produced.
A summary of some of the prior art valve devices is as follows: Unger et al. U.S. Pat. No. 6,408,878 (Unger) teaches elastomeric valve and pump systems wherein a second elastomeric layer is bonded onto a top surface of the first elastomeric layer such that a control channel forms in the second recess between the first and second elastomeric layers and the first elastomeric layer is positioned on top of a planar substrate such that a flow channel forms in the first recess between the first elastomeric layer and the planar substrate. Unfortunately, Unger suffers from complexity of design and cost of manufacturing. In addition to the complexity of the valve, a control system based on pneumatic actuators has to be connected to the various valves through multiple independent lines, and its multiplexing (required in order to have fewer control lines than actual valves on the devices) has impact on the circuit design and requires accurate pressure control.
A patent to Kellogg et al. U.S. Pat. No. 6,302,134 (Kellogg) teaches a heat-activated wax valve in a microfluidics array. This heat-activated wax valve within microsystem platforms requires numerous microfluidics components such as resistive heating elements, temperature sensing elements, mixing structures, to form these heat activated wax sacrificial valves. Apart from a significant occupancy of surface on the microfluidic circuit, the valve of Kellogg further requires an electronic spindle designed rotor capable of transferring electrical signals to and from the microsystem platforms. The requirements and complexity of the Kellogg valve make it impractical to use within micro-analytical systems. Further, the waste from valve actuation can contaminate samples of interest. In addition, the heat is transferred to the wax initially clogging the capillary by heat conduction. In this manner, the heat is also unavoidably transferred to the chip and to the fluids by conduction and convection. This is undesired in most biological applications where the samples could be significantly degraded by heat.
A further prior art valving systems can be found in Kellogg et al. U.S. Pat. No. 6,143,248 (Kellogg '248). Kellogg '248 teaches a capillary microvalve that requires centripetal acceleration to drive fluid in micro-fluid system. The valving device of Kellogg '248 can only be used in a device having centripetal acceleration and also suffers from difficulty in its manufacture.
Another prior art device Kellogg et al. US2002/0097632A1 (Kellogg Application) discloses a bi-directional flow centrifugal microfluidic devices. The valve within the Kellogg Application particularly provides microsystem platforms for achieving efficient mixing of one or a plurality of fluids on the surface of the platform when fluid flow is motivated by centripetal force produced by rotation. This bi-directional flow system is restricted in its use to mixing systems within centripetally driven micro-analytical systems.
Numerous other prior art devices have attempted to improve upon valving devices for micro-analytical platforms, such as Onishi et al. (U.S. Pat. No. 5,547,472) that teaches a catheter with medicament injection pores; Derand et al. (WO00102737A1) (Derand), which teaches polymer valves. An important feature of the polymers used in the valves of the Derand is that they switch from a swelled state to a contracted state or vice versa in a reversible manner, making the choice of the polymer (and its biocompatibility) restricted to a specific class of materials. In addition, the plug is foreseen to be within a capillary, making the manufacturing of the device more expensive and less suitable for mass production since each valve has to be manufactured and positioned within the circuit.
Larsson et al. (WO99/58245) discloses a microfluidic device where the flow of fluids is controlled by different surfaces of the device having different surface characteristics; McNeely et al. (US 2002/0033193) discloses remote valving for microfluidic flow control, Williams (US 2001/0054702A1) teaches a valve for use in microfluidic structures and Parce et al. (U.S. Pat. No. 6,379,974) teaches microfluidic devices and systems utilizing electrokinetic material transport systems to selectively control and direct the transport of materials. Unfortunately, all suffer from complexity of their control systems, design, reliability, high manufacturing costs and application limited to given type of fluids.
Another approach within prior art devices is shown in Limon et al. U.S. Pat. No. 5,869,002 (Limon) where an analysis card containing two mutually separate chambers separated by a frangible partition that is arranged within the analysis card and made of an absorbent and preferably plastic material for absorbing light energy having at least a predetermined wavelength and converting it into heat energy capable of removing the frangible partition thus causing fluid communication between the chambers. Unfortunately, Limon suffers from several deficiencies. The valve of Limon is restricted to a certain configuration that is not adaptable to numerous micro-analytical platforms. More importantly the light energy required within Limon is of such intensity and duration that alteration occurs to the fluids or sample of interest within the adjoining chambers. To overcome the alteration, Limon et al. teach the use of cavities around the frangible partition, to preserve the liquid or liquids circulating in the analysis card from any premature or excessive heating. The valving device of Limon also suffers from its inflexibility in configuration and lack of adaptability to various micro-analytical platforms such as rotating disks or meso-scale devices. Unfortunately, the configuration required by Limon is not adaptable to an economical manufacturing process.
Another drawback of prior art microfluidic circuits has been the difficulty to reconcile flexibility, in the form of fully programmable and configurable devices, with simplicity, in the form of manufacturing and operation. To regulate the flow of fluids through a microfluidic circuit, valves were provided. Prior art methods either rely on active components that can only be provided in limited numbers for reasons of cost and ease of manufacturing, or on passive components that cannot be actuated independently and additionally may depend on characteristics of fluid or the sample of interest. Many active valve systems in the prior art are also characterized by a control system that has to be physically connected to the device, which is often not miniaturized (like the pressure control assembly of the Topaz Crystallizer by Fluidigm Corporation, San Francisco, Calif.) and therefore increases significantly the device complexity, system integration and portability.
A significant drawback of prior art microfluidic circuits has been the difficulty in the handling of biological samples. Prior art devices suffer from valving components that may contaminate the sample of interest, alter or destroy such sample.
Some of the prior art micro-valves integrated in a microfluidic circuit occupy a large surface of the chip. This is at the expense of the other functional components of the device, making the circuit integration (number of components per unit surface) smaller and therefore the chips more expensive. This need to occupy a large surface detracts from their use within a microfluidic circuit.
Another drawback of prior art microfluidic circuits has been the reliability of valving components. Prior art devices suffer from occasional failure and most importantly the lack of feedback controls to recognize such failures. While this aspect may be neglected in chips with a moderate number of valves, (e.g. chips of small complexity), the need of high integration of microfluidic devices requires a higher reliability than the prior art offers of the basic functional components and in particular of valving devices.
A further drawback of prior art microfluidic valves consists in narrow manufacturing tolerances on geometry, surface properties, choice of materials, and complexity of production process. Increasing the integration scale (number of valves in a device) for a manufacturing process which is either complex or has tight tolerances or both results in a high production failure rate, further driving up the cost of production.
Another aspect particular to microfluidics consists in the required disposability of the valves and the overall circuitry. It is well known in the art that the surface-to-volume ratio increases with decreasing volumes. Since a large fraction of the sample is in contact with the chip and valve surfaces, it also means that the fluids contamination is a bigger issue than in the macro-scale world. To avoid contamination of sample, a valve should be preferably used with a single type of sample, and possibly only once to avoid changes in the sample concentration. A valving method relying on reusable valves is therefore less attractive in most microfluidic applications.
The present invention meets the need for a flexible, reliable and yet a simple means to regulate fluid flow, as well as a variety of other needs.